The present invention relates to telecommunication laser devices in general, and more particularly to external cavity lasers with separate gain and filter sections that can provide broad wavelength tunability.
Much of fiber optic telecommunications uses multiple wavelength channels in the same fiber. Having multiple lanes in the same fiber increases the bandwidth capacity and allows rerouting or adjustment of the network by adding or dropping channels from one fiber to the next. To eliminate inventory and logistical issues of producing and servicing of dozens of lasers of different wavelengths, tunable lasers are now generally used in fiber optic networks. A single laser can provide any one of the channels needed and is frequently set to that channel at the start of deployment. A tunable laser also allows the wavelength to be changed after deployment to change the network topology. Thus tunable lasers are of great interest for multi-wavelength fiber optic networks.
Currently there are three main approaches to making a tunable laser for fiber optic network applications, each of which has some limitations in cost and complexity of manufacture. The most popular widely tunable lasers for fiber optic networks are integrated devices manufactured entirely in InGaAsP that have multiple sections. Typically one section provides the optical gain, another controls the cavity phase, and other sections provide wavelength selective feedback. Since all these sections require different geometries and must be carefully matched for proper operation, the chip yield can be low. Furthermore, with all these different elements, the chip size can be quite large, which in the expensive InGaAsP material system, also translates to higher cost. The filter sections need to have high resolution which in general translates to a large size that can sample many wavelengths of the light. Another approach is to use an external cavity configuration, where a small inexpensive InGaAsP gain chip is used, but the wavelength selective feedback comes from bulk optical components such as gratings or Fabry-Perot filters. In this case the chip cost is low, but relatively expensive other components are needed and all the parts have to be carefully aligned. The third and final conventional approach uses an array of single frequency lasers integrated on one chip, where each of the lasers operates at a different wavelength. Depending on what wavelength is needed, a single element of the array is activated. Some form of a switch or a combiner is then used to direct the light into the single output fiber. Here the complexity of the switch and the need to make many different lasers increases the cost and complexity.
A waveguide filter has been developed that instead of using gratings or couplings between dissimilar waveguides uses a ring that resonates at a set of discrete wavelengths that match the size of the ring. The diameter of the ring determines the spacing between these wavelengths (FSR—Free Spectral Range), and relatively large FSRs needed for tunable laser applications imply very small diameters. Therefore an integrated laser that uses such filters can be much smaller in size than an integrated laser that uses gratings for wavelength selective feedback. The issue is that these rings can be difficult to make in the InGaAsP material and they generally have very small optical modes that is generally incompatible with the mode in the gain section. Nevertheless, such lasers have been demonstrated.
Ring resonators have been fabricated and demonstrated in silicon, which is a much less expensive material system, and widely tunable lasers have been fabricated using an InGaAsP gain element and these ring resonators. However, optical coupling between the ring resonators fabricated in silicon with a very small optical mode and the InGaAsP gain chip with the much larger optical mode has been challenging. The high loss of the coupling has so far translated to poor performance.